[gdal-dev] Overriding IRasterIO in class derived from GDALRasterBand

[Jorge Arévalo]

Hello,

2009/8/12 Mateusz Loskot <mateusz at loskot.net>:
> Frank Warmerdam wrote:
>> 2009/8/9 Jorge Arévalo <jorge.arevalo at gmail.com>:
>>> Then, my point could be, when creating the dataset (Open static
>>> method), perform a query to get all the blocks that cover the
>>> extent of the raster (the extent is in RASTER_COLUMNS table) and
>>> store them in any place (A new class attribute?, flush the data on
>>> a tmp file?). Then, override IRasterIO in Dataset to get the data
>>> fetched in the extent query, instead of perform a new call. And
>>> override the RasterBand::IReadBlock method too.
>>>
>>> It will supppose a change of perspective, but it will improve the
>>> performance, of course.
>>
>> Jorge,
>>
>> I'm afraid I have not quite followed the discussion of the IRasterIO
>> implementation, but I find the above statement quite alarming. It is
>> imperative that we *not* end up reading back all blocks if only a few
>> are requested.  Imagine a 100GB image split up into blocks in a
>> table.  If an application using GDAL is run and only needs a small
>> area of the file it is important that the blocks not requested never
>> be pulled from the DB.
>
> I can only second Frank's comment.
> IOW, general idea is to query minimum (sub)set of tiles requested,
> where request is based on spatial extent (i.e. geographic coordiantes)
> or extent within pixel space of a coverage (specified as range of rows
> and columns of a raster).
>

OK. Then, and correct me if I'm wrong, the point is to query only the
extent asked by the program using GDAL driver. Then, I should query
only the blocks requested in, for example, the IReadBlock method, just
as I did. But Tamas' suggestion was to minimize the number of server's
rounds implementing a kind of cache system at driver's level.

How could I combine both concepts? Fetch only the blocks that cover
the area defined by block offsets (One query for each IReadBlock call,
more server rounds)? Fetch more blocks but not all the blocks covered
by the raster extent, potentially millions of blocks? Which method
should I override, apart from IReadBlock in RasterBand? I suppose I
could override IRasterIO in Dataset or RasterBand, but it implies
taking care of a lot of things.

Thanks in advance

Best regards,
Jorge


> Best regards,
> --
> Mateusz Loskot, http://mateusz.loskot.net
> Charter Member of OSGeo, http://osgeo.org
>